The ability of the 8-hydroxy-5-deazaflavin-dependent NADP+ reductase to catalyze the reduction of analogues of the natural cofactor was examined. Comparison of the kinetic values revealed certain substrate structure-reactivity relationships for the enzyme. Hydrogenase was purified from cell-free extracts of Methanococcus vannielii and some evidence for presence of selenium in hydrogenase was obtained.